What Are Some Common Issues with Oscilloscopes and How Can They Be Fixed?

[Xyius]

Hello!

So first I want to say I have very little experience with oscilloscopes other than using one on a computer program. So realizing this, I took it upon myself to buy a used analog one to learn. I bought it for 20 bucks on ebay and the disclaimer said "Do not know if it works!" I took a shot, its only 20 bucks!

Well I got it, plugged it in, and while I did get a back light on the screen, I didn't get any beam across the screen, it was just blank. I tinkered around with everything thinking maybe the beam might have been positioned off the screen or the trigger wasn't making the beam move across the screen but nothing I did worked. I didn't hear any noise coming from inside.

I have a lot of experience with taking things apart and fixing them. I work on cars and computers so I figured I would give it a shot. So I opened it up (I know TV's have a very large capacitor so I was careful of where I was probing because I assume this is of similar engineering.) Nothing seemed obviously wrong (ie: no broken wires or corroded connections from what I could tell.)

Now here is the part I need some real input on. I roughly know how cathode ray tubes work. I know that an element has current run through it and this creates a cloud of electrons, from there the electrons are accelerated through the glass tube by anodes and onto a phosphorous screen. The element in the back of the CRT was very dim, but I cannot base this observation on anything since it is the first time I have opened an oscilloscope. But a bad CRT would certainly explain the "no beam on the screen" problem! Is there anyway to repair one of these things? Or purchase a new one?

I am basically doing this for fun, as I said I love to take things apart and fix them. So it certainly isn't important for this to be fixed.

Opinions?

 

[vk6kro]

I would start looking for a schematic for it and a service manual. These give "correct" working voltages and it is possible to repair devices just by getting clues from incorrect voltages.

You may be able to check around the deflection plates of the Cathode Ray Tube. The opposite plates should have the same voltage on them for the display to be centered.

There will also be a large positive or negative voltage on pins of the tube or possibly a connection at the side of the tube. This will be something like 2000 volts so a normal multimeter should not be used to measure it, even accidentally while checking the other pins.

At this stage, most of the circuitry you see when you open it is not under suspicion.

If you want to take a guess on what is wrong, look for a high voltage diode close to a transformer. These are special high voltage types and blow up quite easily. If that is what is wrong it would be a cheap repair.

 

[NascentOxygen]

Well I got it, plugged it in, and while I did get a back light on the screen, I didn't get any beam across the screen, it was just blank. I tinkered around with everything thinking maybe the beam might have been positioned off the screen or the trigger wasn't making the beam move across the screen but nothing I did worked.

Before concluding that there is something that needs to be fixed, you should invite someone well versed in the ways of CRO's to try it. Managing to get a line on the screen can be quite tricky if the controls have been fiddled with.

Try it at night, when even a faint line stands out. The horiz control should be set to AUTO. Turn down the vertical gain. Set display to CHOP, and sweep to a relatively slow setting. That leaves you with at least POS and BRIGHTNESS to play with.

 

[uart]

Also make sure that the trigger mode is either set to "auto" or "line" and not "single sweep" (if it has that option).

BTW. I know this advise is a bit late, but in my experience whenever anyone is selling something described "as is, it may or may not work. I haven't tested it", that is code for I have tested it and it definitely *doesn't* work.

 

[Averagesupernova]

Having fixed hundreds of analog scopes (worked for a test equipment company) I can tell you it is unlikely you will fix it without a schematic and quite likely another scope. ALOT of things can cause no trace on the CRT.

 

[Mike_In_Plano]

You'd be surprised how often you get lucky and have a power supply problem - especially old capacitors. Tantalum capacitors love to fail shorted, and you can actually spot them by checking with an Ohm meter - If you read 10 ohms or less across it, it, or one of it's buddies , is most likely have a shorted. Then, you unsolder one end and measure across it again.

Happy hunting

 

[dlgoff]

Some day I'm going to try and find a replacement power transformer for my old Heathkit Oscilloscope.

Averagesupernova!

 

[Mike_In_Plano]

Don,

I've found in many good quality transformers, there's a buried thermal cut out for protection. These fail more often than the windings, and at times, the leads to the cut out will be exposed, where you can jumper them.

In that case, I just rig an external cut off at say 110C, insulate it with Kapton, and bond it at the corner of the core and windings. Also, Chatem distributes tiny cut offs that can slip between the windings...

- Mike

 

[dlgoff]

Don,

I've found in many good quality transformers, there's a buried thermal cut out for protection. These fail more often than the windings, and at times, the leads to the cut out will be exposed, where you can jumper them.

In that case, I just rig an external cut off at say 110C, insulate it with Kapton, and bond it at the corner of the core and windings. Also, Chatem distributes tiny cut offs that can slip between the windings...

- Mike

Thanks for the suggestion Mike. It's been many years since I "determined why" the transformer had lost a set of windings, but now I'll be going to get it out of storage.

I'm hoping Xyius doesn't see this as highjacking his thread but as an oscilloscope problem solver thread.

 

[Averagesupernova]

dlgoff, are there any semiconductors in that scope? By the looks of it that isn't a real fast scope with no BNC connectors for inputs. Here is a scope that I have: http://www.oldtestequipmentarchives.com/item.htm?item=77
I believe it has ONE semiconductor diode in it. The rest tubes. Pretty basic unit and certainly not very fast.

 

[dlgoff]

dlgoff, are there any semiconductors in that scope? By the looks of it that isn't a real fast scope with no BNC connectors for inputs. Here is a scope that I have: http://www.oldtestequipmentarchives.com/item.htm?item=77
I believe it has ONE semiconductor diode in it. The rest tubes. Pretty basic unit and certainly not very fast.

I don't think there are even any diodes in this old scope. Here's a schematic of it but I'll need to get it out of storage to make sure it's the correct model.

My first kit was a Sencore VOM. Want to trade scopes?

 

[Averagesupernova]

Probably not up for trading. It's been a year or so since I tried the old scope but it was working fine then. Of course, fine is a relative term. Over the years I have gotten rid of a handful of things I wish I had kept. I considered selling the scope but am now glad I didn't. Got rid of an old VTVM at a swap meet most likely for little or nothing. Wish I had it back.

 

[dlgoff]

Over the years I have gotten rid of a handful of things I wish I had kept. I considered selling the scope but am now glad I didn't. Got rid of an old VTVM at a swap meet most likely for little or nothing. Wish I had it back.

I hear you.

I just like looking at old electronics. I presently have a project cleaning up a box of really old tubes and making some sort of "collection presentation". But this is getting off topic.

Happy scoping